HIV-1 replication requires binding of the host cell protein cyclophilin A (CypA) to the viral capsid. Genetic or pharmacologic inhibition of the CypA-CA interaction results in impaired HIV-1 replication in human T cells, yet the mechanism by which CypA promotes HIV-1 infection is unknown. Recent studies suggest that CypA protects the incoming core from intrinsic host restriction in target cells, yet the requirement for CypA is independent of the recently identified host restriction factor TRIMSa. Evidence for a target cell-dependent requirement for CypA exists in the form of HIV-1 mutants capable of replicating in .the presence of cyclosporin A (CsA). These mutants contain substitutions in the CypA-binding loop of the viral CA protein. In some, but not all, human cell lines the replication of these HIV-1 mutants actually requires the presence of CsA. We hypothesize that as-vet unidentified human cell proteins determine the consequences of CypA binding to the HIV-1 capsid for virus infection. In this R21 application, we propose experiments to test this hypothesis and identify the relevant host factor(s). In Specific Aim 1, we will determine whether the cell- specific phenotypes are consistent with the expression of dominant restriction factors by studying heterokaryons generated from cells that differ in their requirements for CypA-CA interactions in HIV-1 infection. In Specific Aim 2, we will identify the relevant factor(s) by a combination of candidate gene analysis and screening of specific cDNA libraries. These studies will help elucidate the mechanism by which CypA promotes HIV-1 replication and may reveal novel strategies for the treatment of HIV infection. [unreadable] [unreadable] [unreadable]